Various applications require the use of magnets designed to create a particular magnetic field, both in terms of the field pattern and strength. One such application is a proximity sensor for detecting a passing ferromagnetic article, such as a gear tooth. Conventional proximity sensors include a magnet and an integrated circuit having a Hall device. The integrated circuit is positioned in the magnetic field created by the magnet. In use, the Hall device generates an electrical signal related to the strength of the magnetic field normal to the plane of the Hall device. Thus, as a ferromagnetic article moves relative to the Hall device and the strength of the magnetic field changes, the electrical signal generated by the Hall device changes.
One way of tailoring the pattern and strength of the magnetic field provided by a magnet structure in order to provide suitable peak and open circuit characteristics is to use a magnetically permeable concentrator. The concentrator is positioned in close proximity to the magnet and effectively concentrates the magnetic field as a function of its material, geometry, and spatial relationship to the magnet.
In one such magnet structure, a magnetically concentrating steel plate is bonded between two semicircular pieces of sintered Samarium Cobalt (SmCo) magnetic material. The resulting structure may then be machined to provide a desired form factor for use in a particular application. For example, some conventional proximity sensor packages require the magnet structure to have a truncated semicircular cross-section.
The above-described magnet structure suffers certain drawbacks. In particular, costly bonding and aligning processes are required to assemble the magnet and concentrator and, even then, tight position tolerances may be difficult to meet. Further, over time, the concentrator may move relative to the magnet, thereby adversely impacting the device performance. Additionally, there is no easy way to "fine tune" the resulting magnetic field without experimenting with various concentrator materials, geometries, and placement relative to the magnet.